1. Field of the Invention
This invention relates to a transmission power control apparatus for a mobile communication system, and more particularly, to a transmission power control apparatus for controlling a transmission power of radio signals taking respective radio communication qualities of a forward channel (a channel direction from a radio base station to a mobile terminal) and a reverse channel (a channel direction from a mobile terminal to a radio base station) in a mobile communication system, in which the code division multiple access (CDMA) system is used for a communication between a radio base station and a mobile terminal, into consideration.
2. Description of the Related Art
One example of conventional device conguration related to a transmission power control in a radio base station of a mobile communication system using the CDMA system is disclosed in a literature, "An Overview of the Application of Code Division Multiple Access (CDMA) to Digital Cellular Systems and Personal Cellular Networks."
FIG. 11 is a block diagram illustrating such conventional device configuration, and its control operation is shown in FIGS. 12 and 13. FIG. 12 is a flowchart illustrating a conventional control operation of the output signal power for the reverse channel and FIG. 13 is a flowchart of a conventional control operation of the output signal power for the forward channel respectively.
In a transmission power control, two types of transmission power control are involved. One is a transmission power control of radio signals for the reverse channel from the mobile terminal and another is a transmission power control of radio signals for the forward channel from the radio base station, and in the conventional transmission power contorl, these two types of transmission power control are performed independently each other.
The conventional transmission power control of radio signals for the reverse channel is described below with referring to FIGS. 11 and 12.
In the radio base station, a reception field strength of received radio signals of the reverse channel is measured by a digital demodulating section 1101 (Step 1201), and then it is reported to a reception field strength judgment section 1104. In a decoder section 1102, an error of received data is detected (Step 1202), and then the result is reported to a reverse channel error rate judgment section 1103.
In the reverse channel error rate judgment section 1103, the reported result of the error detection is stored for a certain period (for example two sec), and then a frame error rate is calculated based on the stored results (Step 1203 to 1205). In addition, in the reverse channel error rate judgment section 1103, an expected frame error rate has been set previously and an expected reception field strength has been calculated based on the expected value of the frame error rate. Furthermore, in the reverse channel error rate judgment section 1103, the value of the expected reception field strength is modified based on the calculated frame error rate and new expected reception field strength is set (Step 1206), and then it is reported to the reception field strength judgment section 1104.
In the reception field strength judgment section 1104, when receiving radio signals from the mobile terminal, the expected reception field strength previously reported by the reverse channel error rate judgment section 1103 is compared with the reception field strength of the radio signals from the mobile terminal (the reverse channel) which has been measured and reported by the digital demodulating section 1101 (Step 1207). If the reception field strength is greater than the expected reception field strength, an instruction to decrease the transmission power of the mobile terminal is transferred to the mobile terminal by means of radio signals of the forward channel from the radio base station (Step 1209). If the reception field strength is smaller than the expected reception field strength, the instruction to increase the transmission power of the mobile terminal is transferred to the mobile terminal in the same manner (Step 1208).
The transmission power control for the forward channel is described below with referring to FIGS. 11 and 13.
In the mobile terminal, a frame error rate of the received radio signals of the forward channel is measured. Then, the result of measurement is reported to the radio base station on the reverse channel at certain intervals (for example, two sec). In the radio base station, the decoder section 1102 obtains a frame error rate of radio signals of the forward channel which has been measured by the mobile terminal set in the received data (Step 1301), and then reports it to an output power control section 1106. The output power control section 1106 stores the value of transmission power of the current radio base station, and when a frame error rate is reported by the decoder section 1102, it increases or decreases the transmission power by controlling an amplification adjustment section 1107 according to the value of the frame error rate so that the frame error rate stays in a certain range (Steps 1302 to 1303).
In Japanese Non-examined Patent Publication No. 7-030482, there is disclosed a technology that a radio base station measures a frame error rate of radio signals of a reverse channel to control a transmission power of a radio base station and the mobile terminal measures a frame error rate of radio signals of a forward channel to control a transmission power of the mobile terminal.
In a transmission power control in conventional technologies, a power of radio signals transmitted from the radio base station is controlled independently of a control of a power of radio signals transmitted from the mobile terminal.
In a mobile communication using the CDMA system, a channel is formed by a spread code modulation using the same frequency, whereby orthogonality between the codes is insufficient and therefore an interference easily occurs between channels not only for different cells but also for the same cell. Particularly, the reverse channel (from the mobile terminal to the radio base station) is effected by an interference level fluctuation or a deviation caused by a fading or a difference of propagation distances.
If a radio communication quality of one of channels of the forward channel and the reverse channel is degraded, it is sufficient for a radio communication quality of a channel in the other direction to have the same degraded level of communication quality, and it is better to cope with this occasion by moving a position of the mobile terminal or performing a hard-over operation or the like from a viewpoint of reducing interference in the case of a mobile communication adopting the CDMA system. In the transmission power control in the conventional technique, however, a channel in one direction is controlled independently of a channel in the other direction, and therefore a transmission power of a channel whose radio communication quality is not degraded is kept without any changes, whereby radio signals may be transmitted with an excessive power in some circumstances.
Accordingly, if a radio communication quality of either a reverse channel or a forward channel is degraded and it is not improved, there is such a problem in a mobile communication adopting the CDMA system when applying a transmission power control in the conventional technology that excessive interference is given to a communication between another mobile terminal in the same cell or in an adjacent cell and the radio base station.